The time-synchronization procedure of a conventional synchronization system includes five steps:
(1) Delay measurement,
(2) Offset calculation,
(3) Notification,
(4) Confirmation, and
(5) Synchronization,
each of which is implemented in a series of operations (e.g., Patent Literature 1).
A detailed description thereof is given below.
In the (1) Delay measurement step, a communication device (master apparatus) that holds a reference time for synchronization measures a transmission path delay to another communication device (slave apparatus).
This is because the measurement of a transmission path delay helps to implement a more accurate synchronization.
In a connection configuration of a star type, a tree type, a line type, or a mixed type thereof, there are one or more slave apparatuses located at the terminal ends of the master apparatus.
Every slave apparatus belongs to one of routes leading to the terminal slave apparatuses.
The line type is a connection configuration involving relays performed by slave apparatuses having two or more ports and a function to relay a frame between the ports.
The master apparatus transmits a frame for measurement (hereafter, “Measure frame”) to a terminal slave apparatus on each route so as to measure a transmission delay.
The Measure frame is addressed to a terminal slave apparatus and transmitted on each route.
The terminal slave apparatus, upon receipt of the Measure frame, transmits a response frame (hereafter, “MearueAck frame”) for measurement by return.
All the slave apparatuses on the route through which the Measure frame passes relay the Measure frame.
In that process, each slave apparatus records the arrival time of the Measure frame and the arrival time of the MeasureAck frame, calculates the time difference (“roundtrip time tr”), and records the calculated roundtrip time tr.
The master apparatus calculates the roundtrip time tr based on the transmission time of the Measure frame and the arrival time of the MeasureAck frame.
The terminal slave apparatuses calculate the roundtrip time tr based on the arrival time of the Measure frame and the transmission time of the MeasureAck frame.
In the (2) Offset calculation step, the master apparatus calculates time (“offset value o”) for the slave apparatus, based on the roundtrip time tr and time ts being the terminal time of the master apparatus, which is from the transmission of a frame for synchronization (hereafter, “Sync frame”) until a synchronization time point. The time (“offset value o”) for the slave apparatus is time from the slave apparatus receiving the Sync frame until the synchronization time point.The calculation formula is: o=ts−(tr/2).
In the (3) Notification step, the master apparatus distributes the offset value o to each slave apparatus.
The master apparatus transmits a frame for distributing the offset value o (hereafter, “Offset frame”) on the same route as that of the Measure frame to distribute the offset value o to each slave apparatus.
Each slave apparatus calculates time (“delay time os”) to the synchronization time point based on the received offset value o and the roundtrip time tr recorded therein.The calculation formula is: os=(tr/2)+o. 
In the (4) Confirmation step, the master apparatus instructs each slave apparatus to use the distributed offset value o.
Specifically, the master apparatus broadcasts a frame for confirmation (hereafter, “Update frame”) to notify all the slave apparatuses of the use of the offset value o.
In the (5) Synchronization step, the master apparatus notifies each slave of synchronization timing.
Specifically, the master apparatus broadcasts the frame for synchronization (hereafter, “Sync frame”) to notify each slave apparatus of the synchronization timing.
Each slave apparatus uses the delay time os which is the time until the synchronization time point, based on the time point when the Sync frame is received, so as to obtain the synchronization time.